Filament Scale for Spannerhands Holder

As part of adopting Octoprint Filament Scale, I wanted to be able to weigh spool housed in a spannerhands holder.

https://github.com/techman83/Octoprint-Filament-Scale

Inspired by and built upon Victor's excellent work.

https://hackaday.com/2018/08/10/3d-printers-get-a-fuel-gauge-adding-a-filament-scale-to-octoprint/